The rotary output of an internal combustion engine has superimposed torsional vibrations caused by the forces produced by the firing of its individual cylinders. The frequency and amplitude of these torsional vibrations are a function of the number of cylinders the engine has and the engine speed. As is known in the art, the amplitude of the torsional vibrations of a four cylinder engine are greater than the amplitudes of the torsional vibrations produced by a six or eight cylinder engine. In a like manner, the amplitude of the torsional vibrations at low engine speeds are greater than the amplitude of the torsional vibrations at high engine speeds.
In automotive vehicles having manual transmissions, a torsional vibration damper is conveniently incorporated into the clutch assembly to absorb or neutralize these torsional vibrations. In vehicles having automatic transmissions, fluid couplings or torque converters have been used to effectively absorb these torsional vibrations and vibration dampers were unnecessary. However, to increase fuel economy, a lock-up clutch is currently being used on many vehicles which under predetermined operating conditions of the vehicle, lock and provide a direct coupling between the output of the engine and the input to the transmission. When the lock-up clutch is engaged, the torsional vibrations of the engine's output are directly coupled to the transmission.
To eliminate this problem, Ling in U.S. Pat. No. 4,422,535 discloses a damper assembly for an automatic transmission in which the driven member is resiliently connected to the driving member by a plurality of springs arranged circumferentially to absorb the torsional vibrations. Friction members disposed in opposite sides of the driven member provide a degree of frictional damping of driven member to dissipate undesirable resonant vibration energy between the engine and the transmission.
The problem with this type of damper is that the friction between the driving member and the driven member is a constant and is selected to produce a frictional lag to dissipate energy stored in the springs during high amplitude torsional vibrations. Unfortunately, this higher frictional lag also couples a portion of the torsional vibrations to the transmission decreasing the efficiency of the drive train and resulting in an undesirable and often perceptible torsional vibration of the transmission and drive train.
The invention is a vibration damper assembly which introduces a lash into the friction between the rotary input member and the driven member and provides a low friction controlled lag for small torsional vibrations and a high friction controlled lag for torsional vibrations having amplitudes which exceed predetermined limits.